The Induction Of Dissolved Organic Matters On The Chlorothanoil Photodegradation

Chlorothalonil(CT)is a broad organic disinfectant containing chlorine,which was widely used in diseases prevention and control for vegetable,fruit,beans,rice,wheat and other crops.CT has attracted widespread attentions due to the large amount of production,usage and hardly degradation with a long half-life in the environment.The biodegradation,hydrolysis and photodegradation are three main pathways of CT degradation;And photodegradation is the most important way for pesticide degradation.The effect of dissolved organic matter(DOM)on pollutants chemical behavior in the environment has been on fire with DOM widely presence in natural water.In recent years,researchers have found that DOM could promote the CT degradation,but the mechanism is not clear.Therefore,the effects and mechanisms of DOM on CT degradation are very important to understand environmental chemistry of organic chlorinated pesticides in the environment.Our object is to systematicly study the processes and mechanisms of chlorothalonil in the photochemical conversion by DOM-mediated as well as various water environment factors that affecting CT photodegradation.Meanwhile,the humic acids(HA)and fulvic acid(FA)were uesd as DOM models extracted from Dianchi Lake.On the basis of cleared structure and physico-chemical properties of the DOM,the active substances participated in DOM-mediated CT degradation and its contributions were analysed.The main active substance from DOM was explored by CT photodegradation product to derive CT photodegradation pathway during DOM-mediated.Research results are as follows:(1)Chlorothalonil could be hardly removed by hydrolysis or volatilization process in dark conditions.Chlorothalonil can absorb 290 nm～340 nm light and be slowly degraded(0.0171 h-1),which obeyed pseudo-first-order kinetics.Chlorothalonil itself shield of light effects and the limited selected light energy for experiment can decrease the photodegradation rate in high concentration chlorothalonil.3CT*has a main effect on the direct photolysis of chlorothalonil with 80%,however,the ROS(·OH、1O2 and H2O2/O2·-)also could participate the degradation of chlorothalonil.Chlorothalonil solution is not stable when pH>9 because the cyano could be hydrolyzed to form amide.Chlorothalonil removal rate increased with raising the pH values.Chlorothalonil degradation could slow down with the increasing of dissolved oxygen concentration,due to the 3CT*quenching process by dissolved oxygen.(2)Chlorothalonil photodegradation rate could be obviously promoted by DOM mediated.Compared with HA(0.0833 h-1),FA could be irradiated to produce more 3FA*and·OH with higher humification and oxygen-containing functional groups,leading to faster CT degradation rate(0.0833 h-1).3HA*/3FA*,1O2,·OH and H2O2/O2·-participate in the degradation of chlorothalonil in the order of 3DOM*>1O2>·OH>H2O2/O2·-.(3)The promotion of Chlorothalonil photodegradation rate by DOM could decreased with the increasing of DOM concentration.DOM combined with chlorothalonil would reduce the effects of pH value on chlorothalonil photodegradation,however,chlorothalonil degradation rates increased as the pH increased.The increasing ionic strength could prevent electron transfer between DOM elements and extend the life of 3DOM*,resulting in 3DOM*concentration increasing in the solution,which could make the photodegradation of chlorothalonil accelerated.3DOM*plays a major role during the CT degradation process and 3CT*is the precursor for CT during degradation,therefore,the process of ROS generation by dissolved oxygen quenching 3DOM*could inhibit CT photodegradation with the increase of dissolved oxygen,even suppressed.(4)DOM just could change CT degradation rate rather than degradation products.The main products of CT photodegradation are 2,4,5-trichloroisophthalonitrile and 4-hydroxy-chlorothalonil.